In Flight Question

A stranger was seated next to a little girl on the
airplane when the stranger turned to her and said,
'Let's talk. I've heard that flights go quicker if you
strike up a conversation with your fellow passenger.'
The little girl, who had just opened her book, closed
it slowly and said to the stranger,
'What would you like to talk about?'
'Oh, I don't know,' said the stranger. 'How about
nuclear power?' and he smiles.
'OK, ' she said. 'That could be an interesting topic.
But let me ask you a question first.
A horse, a cow, and a deer all eat the same stuff - grass - . Yet a deer excretes little pellets, while a cow turns out a flat patty, and a horse produces clumps of dried grass. Why do you suppose that is?'
The stranger, visibly surprised by the little girl's
intelligence, thinks about it and says,
'Hmmm, I have no idea.'
To which the little girl replies, 'Do you really feel
qualified to discuss nuclear power when you don't know shit?